A common problem in silicon on insulator (SOI) devices is passivation of the sidewalls. Since the sidewalls are not &lt;100&gt; silicon, they will often have an inherently somewhat lower threshold voltage than the plane in which the primary active devices are constructed, and thus passivation of the sidewalls is particularly difficult.
Moreover, silicon on insulator circuits are particularly attractive for radiation-hard circuit applications which must be extremely resistant to single event upset. Under total dose irradiation, the fixed charge density of the dielectric interface does not stay at the as-manufactured level, and therefore it is particularly desirable that the parasitic transistors along the sidewalls of the mesa be very far from inversion, i.e. very far away from their threshold voltages.
Heavier channel stop doping on the sidewalls of the mesas would obviously help to accomplish this, but to date, there has been no fully satisfactory way to achieve this. Since the sidewall regions are nearly vertical, they are difficult to dope with implantation steps. Moreover, there has been heretofore no convenient self-aligned sequence of steps to leave the sidewall regions exposed.
The most popular way to form the sidewall channel stops has been to introduce a heavy doping, patterned by the mesa mask, and drive that dopant to achieve a substantial lateral diffusion before the mesa etch is performed. However, this has the disadvantage of requiring long furnace times early in the process, and also results in a device where the threshold voltage of the parasitic sidewall transistors in the finished device is critically dependent on the time and temperature of this furnace step. Moreover, due to the concentration gradient implied by the diffusion equations, such lateral diffusion techniques will always produce a net concentration in the sidewalls of the mesa which is lower than the original maximum net concentration in the silicon between mesas.
Doping of the mesa sidewalls is an important step in the SIMOX (Separation by Implantation of Oxygen) fabrication process. It is essential that the dopant concentration is high to achieve good radiation hardness. Presently, as shown in U.S. Pat. No. 4,753,896 assigned to the same corporation, sidewall doping is done prior to the high temperature (1000.degree. C.) sidewall oxidation which drives out the boron from the edges. This tends to cause coupling along edges during radiation causing higher leakage current.